Ann Darrow O'Brien was the young, smart, and wealthy daughter of an eminent scientist and explorer who, at the start of the series, had disappeared two years ago and left his daughter and her assistant Akiko Oki living in his palatial Bay Area estate. To the surprise of many, the young Ann had been keeping the estate solvent. But in the first story, Ann's amoral half-sister Oniko (who left the family 15 years earlier to join a mysterious criminal enterprise) and her two henchmen enter the estate to take it over. The sisters have a brawl in an unused dimensional transportation chamber, and Ann accidentally hits a button that kicks off a "life form retrieval beam." The chamber explodes as it summons the giant, simian genius Axewell Tiberius, who's combating an unknown, apelike enemy. The henchmen escape with Oniko and the still-alive head of that enemy, while Ann and Akiko befriend Tiberius and give him a place in the estate's library.

O'Brien discovers that the blast in the chamber had released a mysterious radiation that affects both her and Oniko. Immediately after the explosion, her eyes turn orange -- the same color as Tiberius's. The next night, she needs almost no sleep to feel refreshed. The next day she finds that her sneakers no longer fit her feet: "I was growing faster than a teenager!" She performs better and better at physical tasks, lifting tons of free weights and at one point making a 60 mile run in 30 minutes without much effort. She frets a little about the ongoing growth spurts (wasting plenty of money on clothes that become too small in 24 hours), but finally settles in at seven feet tall and 250 pounds.

Her new strength, size, and knowledge (from working alongside Tiberius) transform O'Brien from an estate manager to a renowned and fearless adventurer. She joins Tiberius in a series of 1960s-inspired adventures, only a few of which are dilineated in the comics. (Author/artist Adams makes references to more adventures, like the "Shrieking Bin-Yak," which are never seen by the readers.) Her eventual goal: Help Tiberius find a way to his home world dimension.

Named for the classic "damsel in distress" hero of King Kong, "Ann Darrow," O'Brien quickly grows up from a smart-but-meek estate manager to a confident adventurer. She often beats Tiberius to the punch and plunges into their dangerous situations; she seems to take more pleasure out of a knock-down brawl or chase then her cohort. She's generally gregarious but obviously doesn't suffer fools; when a reporter asks if she has a sexual relationship with Tiberius, O'Brien needs to be restrained from beating the guy up. While obviously beautiful (both President Bill Clinton and the evil Shrewmanoid have expressed a longing for her), she's a bit awkward in dealing with her massive size. In a dream where she gets blasted by a growth ray and accidentally knocks over half of San Francisco, O'Brien cries that she'll "never have a boyfriend." For his part, Adams has never introduced a romantic foil for O'Brien.

O'Brien's powers have never been carefully explained, possibly because creator Adams is more interested in telling the monster/sci-fi tales than bogging down in details. She has superhuman strength, unusual speed (although not on the level of a Quicksilver or the Flash), and is extremely hard to hurt. In a cross-over with Gen 13, she was able to defeat an evil version of Caitlin Fairchild with some ease - she had to hold back some of her strength so as not to kill her. It's unclear if her powers have changed since her origin, as Adams has dropped some clues that could be red herrings. In Monkeyman and O'Brien #3 she is bitten by the Shrewmanoid, bleeds, and then heals within 5 seconds. No one had noticed this power before; Tiberius is inspired to "run some tests." Over the course of the five years Adams drew her, O'Brien changed physically, becoming more muscular, sporting wilder hair, and developing a more chiseled, "supermodel"-looking face. It's probable that these were just changes in Adams' artistic style.